A filter device with a plurality of recess plates configured as plate stacks and filter chambers, which are respectively configured between adjacent recess plates of the stack, is known from U.S. Pat. No. 4,844,803 A. In the filter chambers, substantially in the respective separation plane between the recess plate and the associated counter plate, one or two filter cloths and a rubber elastic membrane are disposed depending on the configuration of the filter device. During filtering operations of the known filter device, the filter chambers are supplied through filling shoes with the pressurized suspension, which is to be filtered, and the suspension is pressed through the filter cloths. Thus, the solid content of the suspension is deposited on the filter cloth as a filter cake. The filtered suspension from which the solid content is purged flows out from the filter chambers as a filtrate below the filter cloth substantially without pressure.
After filtration, the one or two movable membranes are acted on by pressurized air or another gaseous medium at a gas pressure between 4 to 16 bar. Whenever a leakage occurs in one of the membranes of the known filter device, the pressurizing medium percolates into the cake side and discharges unperceived from the filter device together with the filtrate. Where the leakage as such may easily be recognized by monitoring the fluid flow of the pressurizing medium supplied to the filter device, it is merely impossible to identify the actually affected membrane, in particular where the filter device has a large number of filter chambers.
Methods and devices for detecting a leaking membrane are suggested in DE 299 16 427 U1 and in DE 203 07 912 U1: For each filter chamber, a fluid flow sensor is inserted into the duct for the pressurizing medium. In case the pressurizing medium still flows into a chamber, where it has stopped flowing into the others, a leakage of the respective chamber and its membrane is detected.
During squeezing the filter cakes of the known filter devices, values of fluid flow of the pressurizing medium into the respective squeezing chambers spread over a large interval due to decrease in pressure over the common feed pipe outside the filter chambers, due to heterogeneous load of solid content inside the chambers or due to soiling of the respective ducts. Monitoring the fluid flows thus leads to false positive detection of leaking membranes or, more important, to missing actual leakages.
JP 10 296009 A discloses a device and method for the determination of leaks in membrane filter plates, relying on the determination of a leak flow. It suggests a shut-off valve by means of which the pressure chamber can be closed so that no further pressure fluid can enter the chamber.